1. Field of the Invention
The present invention relates to a drive apparatus for a solid-state image pickup device.
2. Description of the Prior Art
Solid-state image pickup devices have been used in a variety of applications such as an image sensor in a facsimile system or the like. The output characteristics of the image sensor linearly change in accordance with changes in light intensity at a photosensor constituted by photodiodes or the like. For example, when a solid-state image pickup device is used for detecting a focusing state in a camera, such a device must cover a wide sensitivity range from a low luminance of an object to be photographed to a high luminance thereof. For this reason, the storage time interval of a charge to be photoelectrically converted must be controlled to cover a wide dynamic range. Even if the luminance of the object changes, the output level of the charge must be kept substantially constant. Thereafter, signal processing for focus detection is generally performed. Furthermore, in a solid-state image pickup device of this type, in order to equalize noise components caused by a dark current and a low transfer efficiency and eliminate the influence of noise components, a shift register is preferably subjected to continuous transfer operation. For operation in such a manner, detection is performed to ascertain whether an average value of signal charges stored in the photosensor has reached a reference level. The stored charges must be stored in the shift register. However, the storage timing of the charges in the shift register depends upon the amount of light received by the image sensor. The storage timing is independent of the transfer clocks of the shift register. If storage is required, the clock level of the transfer clock is not always set in the charge storage enable state, so that charge storage cannot be performed in this state. In a conventional drive apparatus for a solid-state image pickup device, timing of a storage pulse (to be referred to as a shift pulse) must be matched with that of a transfer clock of the shift register. As described above, when the image pickup device is used for detecting a focusing state of the object, if an object has a high luminance, the proper charge storage time interval is shortened in the image sensor. When the period of the transfer clock is substantially equal to the charge storage time interval of the shift register, a proportion of a time difference between the actual charge storage time interval upon matching between the shift pulse and the transfer clock and the proper charge storage time interval is increased with respect to the proper charge storage time interval. An error caused by the time difference cannot be disregarded.